Switching mode power supplies provide a means of converting an A/C power source into a D/C power source. The means of converting the power source from A/C to D/C includes a rectifier which blocks the transmission of one half of an A/C waveform. The A/C waveform is placed across a diode within the rectifier add when the waveform is essentially in a first phase the diode conducts current. When the diode is essentially in a second phase the diode does not conduct current. When the diode makes the transition from conducting (i.e., on) to non-conducting (i.e., off), a high frequency ringing current occurs during turn off. This current produces an undesirable signal noise or electromagnetic interference (EMI). This is especially true of high current power supplies. The electromagnetic interference can be one of two types, a) conducted or b) radiated. Conducted EMI is primarily emitted and measured at the A/C input of the power supply and has a relatively low frequency of generally less than 10MHz. Radiated EMI is typically emitted at a high frequency, and is measured at any number of places around the power supply. Generally, power supplies have been shielded to suppress the escaped EMI, however, the shielding has only limited effect depending on the frequency of the EMI. The higher the frequency of the EMI, the less effective the shielding, because the shield capacitively passes the interference to the surroundings.
The D/C current flow generated by the power supply is a result of the rectifier diodes being continually switched on and off. This switching generates noise on the internal nodes of the power supply due to ringing in the circuit. That is, the electrical elements in the power supply have been formed in such a manner as to allow voltage transient signals, from switching the diode from "on" to "off", to propagate through the circuit. The noise propagates into the area surrounding the power supply as electromagnetic interference because of the A/C nature of the ringing signal and the capacitive nature of the power supply with respect to surrounding objects. The transients are a result of the stray impedance inherent in the circuit as a result of the mechanical size of a high current power supply Also, the more current the power supply handles, the larger the transients become and the bigger the EMI problem. The stray impedances, and therefore the EMI, can be suppressed or shifted in frequency but not eliminated because stray impedance is a part of mechanically building the power supply.
Prior art techniques have tried to suppress the problem by putting a discrete inductor in series with the rectifier diode. The inductor acts to not allow the current through the diode to change significantly faster than the switching required by the A/C signal on the secondary winding of the power supply. This is an effective method of suppressing the EMI from the rectifier circuit, however, it has two problems. First, the inductor has its own stray capacitance from being assembled into the power supply. As a result, the EMI will not be effectively suppressed by the inductor because the capacitance will pass high frequency noise. Second, typical ferrite core inductors have core leakage, acoustic noise (low frequency), and voltage regulation problems in addition to and separate from EMI problems.
Both the shielding and the discrete element network solution to the radiated EMI problem are not effective for the same basic reason. This is that high frequency effects in power supplies are generated because of parasitic elements inherent in the power supply design. Changing the basic design of the power supply does not significantly increase control over the parasitic effects, and therefore, control over the high frequency effects is limited.